Signal system including an oscillator for generating clock and/or oscillation signal is essential for modern electronics. For example, an electronic demanding wireless communication capability needs a signal system with an oscillator to implement an RF transmitter, receiver and/or transceiver.
Please refer to FIG. 1 illustrating a conventional signal system 100, such as an RF transmitter of a direct-conversion architecture. The system 100 includes a phase detection circuitry 110, a loop filter 140, an oscillator 160, a quadrature modulator 165 and a power amplifier 174. The oscillator 160 oscillates under control of a control signal sf1 to generate an RF clock CKv1. The quadrature modulator modulates baseband data signals I_data and Q_data by the clock CKv1 to form an RF signal ss1, and the amplifier 174 amplifies the signal ss1 to an amplified RF signal sol. To generate the signal sf1 which controls the oscillator 160, the phase detection circuitry 110 detects a phase difference between a reference clock CKref and the clock CKv1 to form a signal se1, and the loop filter 140 filters the signal se1 to the signal sf1. As shown in FIG. 1, the loop filter 140 is a low-pass filter with a frequency response of a bandwidth f0.
Noises affecting the system 100 include reference noise related to the phase detection circuitry 110, oscillator noise determined by its intrinsic jitter performance and resonator design, and injection noise related to the equivalent phase disturbance induced by injection-pulling effect. For example, nonlinearity of the amplifier 174 will cause undesired harmonics, and the harmonics will cause a frequency of the clock CKv1 to be pulled away from an expected frequency, and/or cause a spectrum of the clock CKv1 to deviate from an expected spectrum. Injection-pulling effect is critical for direct-conversion signal system, because frequency of the signals ss1 and sol are substantially the same as (or very closed to) frequency of the clock CKv1. The bandwidth f0 of the loop filter 140 is usually designed for a compromise between the reference noise and the oscillator noise, but such bandwidth f0 will suffer from considerable injection noise.
There are several kinds of prior arts to mitigate the injection-pulling effect. One kind of prior arts attempts to mitigate the injection-pulling effect by improving isolation between the amplifier 174 and the oscillator 160, but suffers from high hardware cost and layout complexity. Another kind of prior arts attempts to mitigate the injection-pulling effect by expanding the bandwidth f0 of the loop filter 140, but suffers from adverse impact to original loop design strategy, which means designers cannot just focus on how to achieve optimal suppression according to reference noise and oscillator noise. Yet another kind of prior arts attempts to mitigate the injection-pulling effect by applying adaptive interference cancellation, but it must suffer from complicated digital calibration and stringent compensation precision demands.